Powered door systems for use on transit vehicles, elevators and the like typically experience very heavy usage which may place severe demands on the motors that open and close the doors. Such motors are generally undersized due to size and weight constraints. Consequently, motor protection circuits are generally necessary to prevent burn out of such motors.
The friction which the doors experience when they are moved between open and closed positions may increase due to a number of factors including temperature, loss of lubricant, galling of load bearing surfaces, etc. In such applications, the doors may be quite massive and may be required to move rapidly between open and closed positions. This places a heavy demand on the motors to overcome the inertia of the doors when accelerating and decelerating the doors during opening and closing strokes of the doors. The doors may also encounter obstructions that require rapid braking which may be done dynamically, again placing heavy loads on the motors. All of these factors cause heat buildup in the windings of the motors.
In order to prevent such motors from overheating, one prior art approach is to place a temperature sensing element in the winding of the motor. The temperature sensing element may include a switch to open the circuit of the motor winding in the event that a dangerously high temperature is encountered. Another approach is to place a temperature sensor in the windings which has signal connection to a controller for the motor to turn off the current to the motor in the event of a dangerously high temperature. Such approaches generally respond slowly to motor overload conditions due to the time needed for heat to penetrate the insulation on the temperature sensor.